Quality grading of the groundwater supply and integration of selected subsurface water pumped directly into treatment and blending systems has been appreciably neglected for various reasons throughout the industrial revolution and more recently throughout the latter part of the 20th century and the early part of the 21st century. It is currently estimated by reputable sources that fresh groundwater constitutes less than one-third of 1% of the total water supply below and on the planet surface. Most of the fresh water supplies are tied up in the polar ice caps and provide an uneconomical solution to our short-term growing demand. Salt and brinish water are predominant, but the cost of desalination can be cost prohibitive as a result of being energy intensive. In many cases, water treatment technologies have not yet advanced in efficiency to the required price point to make the treatment of such salt and brinish water truly cost-effective. As a result, affordability can be problematic. Currently, desalination can cost approximately 2.5 to 3 times as much as the extraction of groundwater in most instances. Moreover, desalination to service millions of people would require extensive pipeline construction and management to bring fresh water from coastline areas to more inland regions. Complimentary to the need for efficient water transportation are the costs for maintaining such an extensive pipeline network. Hazardous waste then becomes another important issue in any desalination efforts.
From a brief recap of recent coastal historical disasters and construction failures, it becomes obvious that groundwater has some key advantages over fresh surface water and sea water. Less treatment, more localized to existing markets with lower transportation costs than surface water, and lower energy consumption are some of the reasons why so many communities throughout the world rely on groundwater as a precious resource. However, there are growing economic challenges with this resource as well, and as time advances we will likely see its price increase, perhaps dramatically.
Stratified water quality surrounding millions of subsurface wells, i.e. groundwater production wells, is presently an unknown, little explored resource. The reality is that there are vast supplies of clean groundwater and moderate quality groundwater in intermediate and deeper aquifers in the subsurface environments. However, little has been done to identify or understand the distribution of these resources and how they can be extracted at a lowest cost.
When a high-capacity production well is built in the ground it is typically constructed with a well casing having long lengths of perforated pipe called well screens. The long well screens are placed in vertical alignment with surrounding aquifers that produce water. Many of these wells have multiple sections of well screen that are depth-located with the surrounding aquifers. Although pilot or exploratory boreholes can be drilled prior to production well scale-up to identify zones of varying water quality, the hydrogeological data generated from the pilot-hole water quality zone-tests cannot simulate how the groundwater in the scale-up well will be pumped or blended with the well before it reaches the surface, over a long period of time (that being weeks, months and years). Zone tests within the pilot hole are economically limited in terms of testing time that is affordable and hydraulically limited due to the typically much lower pumping rates used for these tests. In the real world, production wells run on a continuous or semi-continuous basis and typically hydraulically engage each aquifer over a larger effective radius and vertical depth. As a result of these cost constraints, pumping time (only six to twelve hours for each zone), in a small diameter hole, and at a pumping rate that is typically less than that of the full scale-up production well, the results are often not comparable to a scale up well's performance. Thus, the window of physical testing and observation available with a pilot hole is quite small compared to the 24/7, 365 days per year use of the production subsurface well. There are instances where a disconnect occurs between the favorable results found in the pilot hole and the potentially unfavorable results from a scale-up well that is directly related to the differences of the hydraulic radius of influence of each pilot hole zone test in comparison to the larger radius of influence of the high-yield production wells. Quantitatively, the pumped draw-down differences between the pilot hole and the production are different, as well as in terms of the Bernoulli forces that depressurize the surrounding aquifers. The differences in formationally directed hydraulic forces between the pilot hole and the production well often lead to water quality results that are very different than expected and very disappointing when the new production well is turned on for the first time; or soon thereafter.
Fundamentally, it is difficult, if not impossible, for the pilot hole zone tests to simulate and reliably predict the zonal water quality and yield contributions that are blindly blended in various combinations inside the production well under greater hydraulic stresses. Plots of XY coordinate water quality discharge data from existing wells in combination with the limited zone test data from the pilot hole provide a fragmented picture of water quality distribution within the subsurface aquifers. When the well is constructed, the flow contribution from each water quality zone that is proportionally weighted against zonal chemistry is most often unknown. Oftentimes, the well is powered on with little thought as to how much water is produced from each zone and the resulting blended water quality.